The effect of improper heat treatment progress on the structure and properties of P91 steel was studied. Results showed that under aircooling conditions, although the normalization temperature is beyond conventional range, the hardness meets the requirements of ASME standard, and the microstructure appear a gradual transition from equiaxed ferrite with carbide precipitates to tempered martensite. In addition ,when normalization temperature was as high as 1150°C and 1250°C, there is an characteristic morphology of coarse martensite laths and some of them are orthogonal to each other. When normalization cooling rate is slowly at 1°C/min, the hardness dropped under the lower limit at reduced normalization temperatures and the microstructure consist of polygonized ferrite and carbide precipitates. SEM and TEM observation indicated that with the increasing normalization temperature the ferrite morphology transforms from coarse polygonized to finer equiaxed, and carbide precipitates not only along boundaries but also within the grain, at the same tine rod shape carbides formed at 1000°C normalization.